The detection of explosives, chemical warfare (CW) agents, biological toxins, and other organic molecules that might affect public safety or the environment is a subject of continuing strong interest in analytical chemistry, driven by threats to civil society and by environmental problems associated with explosives residues and by-products. The requirements of an ideal method include (i) high sensitivity, (ii) applicability to involatile and thermally unstable analytes, (iii) high specificity to minimize the chance of false positives or false negatives, (iv) rapid response times, and (v) no sample preparation or handling.
Ion mobility spectrometry (IMS) has been a common choice for addressing this problem. IMS has the advantage of high sensitivity and speed, but suffers in terms of the other criteria. Mass spectrometry (MS) is widely considered to have the best specificity of any technique applicable to the broad class of explosive, toxic and other compounds, and it is highly sensitive, but mass spectrometry has generally required significant sample manipulation. Another barrier to the use of mass spectrometry is that some of the analytes of interest such as some explosives are non-volatile compounds which are not easily ionized by traditional methods. Although a wide variety of desorption ionization methods is available for the MS analysis of compounds on surfaces, they generally require operation under vacuum conditions. Since traditional desorption ionization methods fail at in-situ explosives detection, the approach usually pursued involves wiping the ambient surface with a special material wipe followed by thermal desorption/gas phase ionization of any compounds picked up from the surface by the wipe. Although this dry sampling/thermal method is widely employed in airport explosive detection systems, it requires manual sample transfer, is relatively slow, and is not ideal for the detection of thermally labile explosives or explosives which have low vapor pressures.
Furthermore, the requirement for sample manipulation is also a disadvantage of solution phase mass spectrometry methods of analysis based on electrospray ionization such as that disclosed in the International Publication Number WO 2005/017936. This is unfortunate because most explosives show high affinities for various anions and can be ionized directly by electrospray ionization or by anion attachment, typically using anions generated by an electrospray. The high electron affinities associated with the nitro- or nitrate functional groups present in the overwhelming majority of explosives in common use means that they readily form negative ions by electron capture. Various electron sources including corona discharge, glow discharge and 63Ni beta emitters have been successfully implemented as ion sources for explosive detection, including the direct detection of explosives in air. An ion source of particular interest is disclosed in U.S. Pat. No. 6,949,741, which exposes a sample to a stream of metastable neutral excited-state species of a carrier gas to form analyte ions. The recently developed DESI method, disclosed in United States Application Publication No. 2005/0230635, is performed by directing a pneumatically-assisted electrospray onto a surface bearing an analyte and collecting the secondary ions generated by the interaction of the charged microdroplets from the electrospray with the neutral molecules of the analyte present on the surface. The ionization of analyte can be either positive or negative depending on the polarity of the high voltage source and the susceptibility of the analyte to the particular reaction process involved. An alternate mechanism can occur with DESI, namely, the impact of electro-sprayed droplets on the surface, dissolution of the analyte in the droplet, and subsequent evaporation by mechanisms well know from ESI. While this is generally viewed as a positive feature, there arise situations where one would like to preclude all but a single ionization process mechanism.
What is needed is a system that provides for a single ionization process mechanism so that the analysis of the analyte interaction with various ions can be studied. Such a single ionization process would desirably allow for fast screening of substrate surfaces for trace quantities of analytes such as explosives, CW agents, biological toxins, and other contraband materials. Such a single ionization process could also find utility in quality control, environmental analysis, food safety, and other areas of commercial interest.